Heterogeneous anionic polymerization process

ABSTRACT

The present invention relates to a heterogeneous anionic polymerization process, catalyzed by a metallic insertion graphitic compound. According to the invention, the alkaline metal chosen for the insertion compound is lithium, and the insertion compounds are of binary or ternary type; in the latter case, the insertion compound may comprise an aromatic hydrocarbon also inserted within the graphitic structure. The invention relates in particular to the homopolymerization of butadiene or isoprene, and to the copolymerization of isoprene-styrene.

The present invention relates, on the one hand, to a heterogeneousanionic polymerization process catalyzed by a metallic insertiongraphitic compound, and on the other hand, to the polymers obtained bymeans of this process.

By polymerization in the context of the present invention, is meant botha homopolymerization of one and the same monomer and a copolymerizationof a monomer and of at least one comonomer, copolymerizable with oneanother. In the first case, the polymerization product is calledhomopolymer and, in the second case, copolymer.

In U.S. Pat. No. 2,965,624 of Dec. 20, 1960, a heterogeneous anionicpolymerization process had already been proposed for the manufacture ofsynthetic rubbers, meeting, in general, the following definitions.

In order to carry out the anionic polymerization in a heterogeneousphase, an appropriate reaction medium is prepared, comprising:

A hydrocarbon solvant; namely, either a saturated aliphatic solvant, forexample, hexane; or an aromatic solvant, for example, benzene; or asaturated alicyclic solvant, for example, cyclohexane;

At least one monomer, dissolved in the aforementioned solvant, with aconjugated dienic structure, for example, 1-3 butadiene or isoprene;

At least one catalyst, in suspension in the aforementioned solvant,consisting of a binary metal insertion compound, comprising a carbonwith a graphitic structure, for example a graphite, and at least oneelement inserted in the aforementioned graphitic structure, namely, analkaline metal.

The alkaline metals, as stated in the aforementioned U.S. Patent asbeing capable of forming binary insertion compounds with the graphiticstructure, were solely sodium, potassium, rubidium, cesium, and notlithium. For, in fact, until recently, as will be shown later, nophysical method was known enabling binary insertion compounds to beobtained with lithium.

In French Pat. No. 1.228.027, issued on March 14, 1960, a heterogeneousanionic polymerization process is described which is similar to theprocess described previously. Although the metals considered, in thisFrench patent, as being capable of forming binary insertion compoundswith a graphitic structure, were generally described as belonging to thealkaline metal group, only the use of potassium was described by way ofan example to obtain the designs of binary insertion compounds. In fact,for the same reasons as those mentioned previously, at the time of thefiling of the French patent under consideration, it was physicallyimpossible to obtain binary insertions with lithium.

The different processes for obtaining binary insertion compounds with analkaline metal, with the exception of lithium, were fully described inthe following document:

1. Paper by Albert Herold in the "Bulletin de Societe Chimique deFrance" of 1955, page 999.

In addition, in French Pat. No. 1.402.947, issued on May 10, 1965, aprocess was described for obtaining binary insertion compounds, inparticular, based on lithium. This process consists, first, in obtainingin a polar solvent, for example, tetrahydrofuran, an addition compoundof the chosen alkaline metal, for example, lithium, and a polynucleararomatic hydrocarbon, for example, naphthalene, and second, in makingthe solution of the addition compounds obtained act on the chosen carbonwith the graphitic structure, for example, graphite.

Although in the specification of French Pat. No. 1.402.947, the claim ismade that the process described enables binary insertions to beobtained, in particular, the graphite/lithium binary insertioncompounds, careful crystallographic and thermogravimetric studies havedemonstrated, since then, that the insertion compounds obtained were, infact, ternary and not binary compounds and could be defined ascomprising, essentially:

the carbon with the chosen graphitic structure, for example, a graphite;

the chosen alkaline metal, for example, lithium, inserted in the crystallattice of the graphitic carbon,

the chosen polar solvant, having served for the preparation of theaforementioned addition compound, also inserted in the crystal latticeof the graphitic carbon; generally, the alkaline metal remains bound tothe polar solvant between the graphitic planes of the chosen carbon.

In other words, according to French Pat. No. 1.402.947, graphiticcarbon/alkaline metal/polar solvant ternary insertions are obtained andnot graphitic carbon/alkaline metal binary insertions. Such a correctionof the interpretation of the results was, in fact, made by the applicantof the aforementioned French patent, in the description of Example 1 inthe French Certificate of Addition published under No. 2.067.543.

The process described in French Pat. No. 1.402.947 thus constitutes ameans of obtaining the aforementioned ternary insertions with a polarsolvent. This means, moreover, has been studied and described morecompletely in the following documents:

2. Paper by C. Stein, L. Bonnetain, J. Gole, in the "Bulletin de laSociete Chimique de France", 1966, page 3166;

3. Paper by M. Rose, M. Prost, J. Gole, in the "Comptes Rendus de l'Academie des Sciences de Paris," 1967, volume 265 C, page 616;

4. Paper by Co-Minh Duc, J. Gole, in the "Journal de Chimie Physique,"1972, volume 6, pages 986.

Regarding the possibilities of obtaining graphitic carbon/alkalinemetal/aromatic hydrocarbon ternary insertion compounds, these areconsidered, theoretically, in the following paper and have, so far, notbeen confirmed experimentally:

5. Paper by I. B. Rashkov, I. M. Panayotov, N. N. Tyutyulkov, in the"Bulletin de la Societe Chimique de France," 1975, volumes 5-6, page1271.

In conclusion, up to a relatively recent date, the previous art,detailed previously, can be summarized as follows:

1. all the graphite carbon/alkaline metal binary insertion compounds areknown, with the exception of those involving lithium,

2. only the graphite carbon/alkaline metal (including lithium)/polarsolvant ternary insertion compounds had been obtained experimentally,with the exception of those including an aromatic hydrocarbon.

These conclusions have been modified by the fact that graphiticcarbon/lithium binary insertion compounds are recently obtained, usingthe technique described in the following document:

6. Paper by D. Guerard and A. Herold, in the "Comptes Rendus de l'Academie des Sciences de Paris," 1972, volume 275 C, page 571.

From then it becomes interesting in regard to the heterogeneous anionicpolymerization of a conjugated diene monomer:

1. to test out these new graphitic carbon/lithium binary insertioncompounds as catalysts,

2. endeavour, with these new compounds, to obtain graphiticcarbon/lithium/aromatic hydrocarbon ternary insertions, and try out thelatter as catalysts or, in case of failure, the corresponding aromatichydrocarbon plus graphitic carbon/lithium binary insertion mixtures.

Thus, in general, the present invention consists of a heterogeneousanionic polymerization process, according to which a reaction medium isformed, comprising at least one monomer having a diene structure, atleast one catalyst comprising a carbon with a graphitic structure and atleast one element inserted in this latter, namely lithium, chosen as thealkaline metal.

This process then enabled a surprising observation to be made.

In general, by choosing lithium as the alkaline metal of the catalyst,in particular, choosing as a catalyst a graphitic carbon/lithium binaryinsertion compound, for example, a graphite/lithium binary insertioncompound, the microstructures of the homopolymers and copolymersobtained are not fundamentally different from those obtained by otheranionic polymerization methods, notably in a homogeneous phase with acatalyst such as n-butyl-lithium.

This constitutes a surprising result, on account of the followingscientific reasoning:

1. it is resonable to believe that the catalytic action of the graphiticinsertion compounds is linked to their capacities of penetrating amonomer, and possibly a comonomer, within the interior of the graphiticplanes of these compounds; the greater the spacing between theaforementioned planes, the earier it is for the monomers to gain accessto the active sites of the insertion compounds and the more likely it isfor the macromolecular chains to grow normally, and conversely,

2. the spacing between the graphitic planes depends on the atomic radiusof the insertion metal; the smaller the latter, the smaller is thespacing between the graphitic planes, and conversely,

3. since the lithium has a relatively small atomic radius, compared withthe atomic radii of the other alkaline metals, this should give themonomer, and possibly the comonomer, difficulty in penetrating theinterior of the graphitic planes of the insertion compound, and thusdifficulties in regards to the growth of the macromolecular chains; but,as is shown experimentally later, it has been found that this is not thecase, providing the appropriate operating conditions are chosen.

In particular, when the reaction medium, according to the invention,includes an aromatic hydrocarbon, with which the diene monomer is alsobrought into contact, macromolecular microstructures are, generally,obtained, these being very similar to those obtained when other anionicpolymerization methods are used, in particular, in the homogeneous phasewith a catalyst such as n-butyl-lithium.

This is an equally surprising result, for the same scientific reasonsthat were given previously.

In addition, still in regard to an aromatic hydrocarbon and a graphiticcarbon/lithium binary insertion compound, in most cases it has beenfound that a ternary insertion compound is obtained "in situ" inside thereaction medium and comprising:

the carbon with a graphitic structure, for example graphite,

the lithium inserted in the graphitic structure,

and the aromatic hydrocarbon, also inserted in the graphitic structure,

and that this ternary insertion constituted, in fact, the polymerizationcatalyst.

The existence of such a ternary insertion compound, inside the reactionmedium, no doubt explains the results that have been obtained anddescribed previously. Moreover, mention can be made of similar resultswhen instead of obtaining the aforementioned ternary insertion compoundin situ, this latter was obtained independently of the polymerizationreaction medium, by bringing the corresponding binary compound intocontact with an aromatic hydrocarbon, according to the method suggested,theoretically, in paper (5), followed by the introduction of the binarycompound this obtained into the reaction medium.

For it is possible to prepare a ternary insertion compound underconsideration by bringing the corresponding binary insertion compoundinto contact with a solution of the insertion hydrocarbon in analiphatic solvant, at a temperature of -20° C to plus 120° C, preferablybetween 5° C and 60° C, for 1 to 30 days, stirring continually.

The aromatic hydrocarbon, and the solvent must be carefully purified, soas to avoid any trace of humidity or of a proton donor compound. This iseffected under vacuum or in an inert atmosphere. The new compounds thusobtained are purified by filtration under vacuum or in an inertatmosphere.

On the other hand, it has been found, with surprise, that whenhomopolymerization of 1-3 butadiene or isoprene is carried out under theoperating conditions previously described, homopolymers are obtained,mostly with 1-4 chains, which gives the product obtained a relativelylow vitreous transition point and good elastomeric properties.

In addition, by compolymerizing isoprene (diene monomer) with styrene(α-olefine comonomer, copolymerizable with the diluent monomer) it isfound, surprisingly, that statistical copolymers are obtained, having astructure which is very close to an alternating structure, whereas itcould be expected that sequential copolymers would be obtained.

This is a basic result, so far as the statistical styrene-isoprenecopolymers, marketed under the name of SBR or GRS, are at presentobtained commercially in emulsion form by the radical method, orexperimentally in solution by the anionic method. The process, accordingto this invention, therefore, opens up a new way of obtaining theaforementioned copolymers, namely, anionic copolymerization in aheterogeneous phase, with all the advantages that this implies, that isprincipally, the fact that contrary to that which is the case in theanionic polymerization processes in a homogeneous phase that werepreviously mentioned, the 1-4 cis microstructure of the diene is notdisturbed substantially.

In addition, the styrene-isoprene copolymers obtained in accordance withthe invention, have a much lower vitreous transition point and farbetter elastomeric properties than that and those of thestyrene-isoprenes obtained at present in emulsion form using radicals.Thus, a styrene-isoprene copolymer prepared according to the invention,comprising a molar styrene chain percentage of 41%, has a vitreoustransition point of -43° C, whereas a copolymer of identicalcomposition, prepared in emulsion form with radicals, has a vitreoustransition point of -6° C.

In short, therefore, it can be stated that the process, according tothis invention, enables numerous presently known polymers and copolymersto be obtained by means of a preparatory method, namely, anionic methodin heterogeneous phase, which is totally different from the preparatorymethods at present used in industry.

In addition, this invention also comprises the following secondarycharacteristics:

1. when a graphitic carbon/lithium binary insertion compound is used,the preferred compound is a binary graphite/lithium compound with anempirical formula of LiC_(6n), with 1 ≦n ≦6; the empirical formula ofsuch a compound can be determined by flame photometry and/orthermogravimetric analysis,

2. the polymerization can equally well be carried out in a liquid orgaseous phase; it can also be mass polymerization,

3. when polymerization takes place in a liquid phase, the reactionmedium comprises, for example, a solvant in which the monomer has beendissolved and in which the catalyst is in suspension; this solvant is,in particular, chosen among the following hydrocarbon solvants, that is,a a straight or branched chain saturated aliphatic solvant, and asaturated alicyclic solvant,

4. when polymerization takes place in solution and the reaction mediumincludes an aromatic hydrocarbon, one can use the latter as a solvant ordissolve the aromatic hydrocarbon in the solvant used, if theaforementioned hydrocarbon is a solid substance.

The aromatic hydrocarbons that can be used, in accordance with thepresent invention, can be classified as follows:

benzene and mono-, di-, tri-, and polyalcoylbenzenes with straight andbranched chains, such as toluene, xylene, ethylbenzene, isobutylbenzene,etc.,

benzenes with an alcoylene substitute such as α-methylstyrene 1-1diphenylethylene,

aromatic hydrocarbons such as biphenyl, triphenyl,

polynuclear aromatic, hydrocarbons, such as naphthalene, anthracene,phenylnaphtalene, etc.

The carbons with a graphitic or lamellar structure which can be used,according to the present invention, all have a crystal lattice similarto, or identical with, that of graphite, well known to the experts. Inthis connection, by way of example, the following substances can becited: graphite, certain graphitic carbon blacks, in particular thoseused for loading rubber, lamp, black, pyrographite, activated carbons,etc.

Still, according to this invention, when a saturated aliphatic oralicyclic solvant is used, in general, it includes 5 to 10 carbon atoms.In this connection, by way of example, the following compounds can becited: pentane, cyclopentane, hexane, cyclohexane, heptane, isopentane,2, 2, 4 isohexane, trimethylpentane, decane, methylcyclohexane, andiso-octane.

In accordance with this invention, at least one of the monomersparticipating in the polymerization reaction has a conjugated dienestructure. These monomers do not include any substituents which couldhinder polymerization, for example, polar groups and, in particular,alkoxy groups. The following compounds can be cited in this connection:1, 3 butadiene, isoprene, 2, 3 dimethyl 1, 3 butadiene, 2 methyl 1, 3pentadiene, piperylene, 2, 3 dimethyl 1, 3 pentadiene, 2 methyl 3 ethyl1, 3 pentadiene, 2 phenyl 1, 3 butadiene, 2, 3 diethyl 1, 3 octadiene.

In accordance with the present invention, the chosen comonomers,copolymerizable with the monomers, can be, in particular, α-olefins suchas styrene and various other alcoyl styrenes.

In the manner known; the polymers obtained, according to this invention,can be loaded, vulcanized, and treated like natural rubbers.

The process, according to this invention, has been tested under thefollowing operating conditions.

First, the monomers and solvants were purified according to theconventional methods used in anionic polymerization, and described, forexample, in the following documents:

7. Paper by I. M. Panayotov and I. B. Rashkov, in the "Journal ofPolymer Science," 1972, Part A1, Volume 10, page 1276,

8. Paper by I. M. Panayotov and I. B. Rashkov, in the "Journal ofPolymer Science," 1973, Part A1, Volume 11, page 2615,

9. Paper by I. M. Panayotov and I. B. Rashkov, in the journal"Makromolecular Chemie," 1975, Volume 175, page 3305,

10. Paper by A. Essel, Q. T. Pham and J. Gole in the "Journal of PolymerScience," 1973, Part A1, Volume 11, page 1851.

Second, the binary insertion compounds of lithium were prepared inaccordance to the method described in publication (6), from groundMadagascar graphite; the binary insertion compounds generally obtainedcorresponded to the empirical formula LiC₁₂. Samples of the catalystthus prepared were weighed and packed under vacuum.

Third, all of the polymerizations, whether homopolymerizations orcopolymerizations were concerned, were carried out under theconventional, anionic polymerization conditions, that is, in the absenceof air and any humidity and in an inert atmosphere. Such experimentalconditions were, for example, given in detail in U.S. Pat. No.2,695,624, and French Pat. No. 1.228.027. Moreover, thesepolymerizations were carried out in glass reactors, provided withfragile joints and sealed under high vacuum. In general, the solventchosen (80 ml) was introduced first into the reactor, then the catalyst,and immediately afterwards the chosen monomer (5 ml). The polymerizationtemperature, in general, lay between -80° C and +30° C, and variationsof the latter enabled the microstructure of the polymers obtained to bemodified.

Fourth, a high resolution NMR analysis was made of the configurations ofthe polymers obtained with the aid of a VARIAN DA 60 IL spectrometeroperating at 60 MHz. The polymers obtained were dissolved in deuteriumbenzene. The method of analysis was that described in the followingdocument:

11. Paper by Q. T. Pham, in "Polymers Letters," 1970, Volume 8, pages723-729.

Fifth, the analysis of the molecular masses of the polymers obtained wasmade with the aid of chromatography on a permeable gel, with a set offive columns of silica marbles known under the name of "Spherosil" withthe references, respectively, of X OA 200, X OB 75, X OB 30, X OA 400and X OC 005. The elution solvant chosen was tetrahydrofuran,circulating in the aforementioned columns at 25° C at a rate of 1 ml perminute. Polystyrenes of known molecular mass were used for calibrationpurposes. The molecular masses of the polymers obtained were calculatedwith the aid of the following equation: log [n] M = 1.693 log V_(e) -1.41, taking the Benoit and Grubisic universal standard as a base, [n]being the intrinsic viscosity of the polymer, M the mean molecular massin number, and V_(e) the elution volume. In this connection, referenceshould be made to the following documents:

12. Paper by H. Benoit, Z. Grubisic, P. Rempp, D. Decker, and J. G.Zilliox published in the "Journal de Chimie Physique," 1966, Volume 63,page 1507,

13. Paper by Z. Grubisic, P. Rempp. H. Benoit, published in the "Journalof Polymer Science," 1967, Volume B5, page 753.

Sixth, the vitreous transition points were obtained by differentialthermal analysis with the aid of a DU PONT thermal microanalyser,provided with a DSC cell and a temperature rise rate of 10° C perminute.

Tables 1, 2, and 3 refer, respectively, to the polymerization ofbutadiene (5cm3 of monomer in 80cm3 of solvant), the polymerization ofisoprene (5cm3) of monomer in 80cm3 of solvant), and thecopolymerization of isoprene and styrene. Table 4 shows the influence ofthe quantity of the catalyst C₁₂ Li on the polymerization time ofisoprene.

                                      Table 1                                     __________________________________________________________________________                    Weight of gra-                                                                phite of bi-                                                             Polymer-                                                                           nary insertion                                                           ization                                                                            compound intro-                                                                        Polymer-  M.sub.w                                                                          Microstructure                                                                         Vitreous                                  time in                                                                            duced into the                                                                         ization    = I                                                                             configurations                                                                         transition                     Catalyst                                                                            Solvant                                                                            hours                                                                              reactor, in g                                                                          yield                                                                              M.sub.n                                                                            M.sub.n                                                                          1-4                                                                              1-2   point in                       __________________________________________________________________________                                                   ° C                     Binary                                                                              cyclo-                                                                  insertion                                                                           hexane                                                                              42  0.0669   15.4 240 900                                                                            1.22                                                                             55 45    -26                            compound                                                                      with an                                                                             cyclo-             51.3         90 10    -96                            empirical                                                                           hexane                                                                             100  0.0994                                                        formula                                                                       of Li C.sub.12                                                                      toluene                                                                            100  0.1024   21.5         88 12    -95                            n-butyl-   (14) Paper by C.A. Vranek, in the                                                                        cis                                                                              trans                                lithium    "Journal of Polymer Science",                                      in homo-                                                                            cyclo-                                                                             1971, Part A1, Volume 9,   42.8                                                                             52.2                                                                             5.0                               geneous                                                                             hexane                                                                             page 2273.                                                         phase                                                                         __________________________________________________________________________     M.sub.n  : mean molecular mass in number                                      M.sub.w  : mean molecular mass in weight                                       I : polymolecularity index                                              

                                      Table 2                                     __________________________________________________________________________                    Weight of gra-                                                                phite of bi-                                                             Polymer-                                                                           nary insertion                                                           ization                                                                            compound intro-                                                                        Polymer-   Microstructure                                                                            Vitreous                                 time in                                                                            duced into the                                                                         ization R.sub.w                                                                          1-4                                                                              1-4      transition                    Catalyst                                                                            Solvant                                                                            hours                                                                              reactor, in g                                                                          yield                                                                              R.sub.n                                                                          R.sub.n                                                                          cis                                                                              trans                                                                            1-2                                                                              3-4                                                                              point in °             __________________________________________________________________________                                                    C                             Binary                                                                              cyclo-                                                                  insertion                                                                           hexane                                                                             168  0.0318   80.0 240                                                                              1.80                                                                             44 35 0  21 -53                           compound                                                                      with an                                                                             toluene                                                                             66  0.0505   53.0 117                                                                              2.88                                                                             64 27 0  9  -64                           empirical                                                                     formula                                                                       of Li C.sub.12                                                                lithium                                                                             cyclo-                                                                             (15) Paper by A. Essel, R. Salle, J. Gole,                                                             73 22 0  5                                in homo-                                                                            hexane                                                                             in the "Journal of Polymer Science",                               geneous    1975, Part A1, Volume 13, page 1853                                phase benzene                       62 30 0  8                                potas-                                                                              cyclo-                                                                  sium in                                                                             hexane                                                                             Publication (15)         22 37 5  36 -41                           homoge-                                                                       neous                                                                         phase                                                                         __________________________________________________________________________     R.sub.n:Radius of mean rotation in number                                     R.sub.w: Radius of mean rotation in weight                               

                                      Table 3                                     __________________________________________________________________________    Experimental Conditions                                                                          Weight of gra-                                                                phite of the                                                                  binary inser-                                                                          Results                                              Initial                                                                             Intro-                                                                             Polymer-                                                                           tion compound                                                                          Percentage                                                                           Total                                                                              Yield with                                                                           Yield with                        percentage                                                                          duction                                                                            ization                                                                            introduced in-                                                                         of styrene                                                                           yield                                                                              respect to                                                                           respectto                                                                             Vitreous                  of styrene                                                                          of sty-                                                                            time in                                                                            to the reactor,                                                                        chains in                                                                            as per-                                                                            styrene as                                                                           isoprene                                                                              transition             No.                                                                              in moles                                                                            rene hours                                                                              in g     moles  centage                                                                            percentage                                                                           percentage                                                                            point in °      __________________________________________________________________________                                                           C                      1  46.6  with  66  0.1908   41    51    43.4   59.2    -43                             isoprene                                                             2  34.3  15 hours                                                                           116  0.1496   11.5  45    15.9   67      -52                             before                                                                        isoprene                                                             __________________________________________________________________________     Note:                                                                         Tests No. 1 and 2 were made with:                                             a total concentration of monomer and of comonomer of the order of 10% wit     respect to the solvant,                                                       a ratio of isoprene to styrene of the order of 1/1.                      

                  Table 4                                                         ______________________________________                                        Solvant  Weight of graphite in g                                                                      Reaction time in h                                                                          Yield                                   ______________________________________                                        cyclohexane                                                                            0.032          168           80.0                                             0.149          24            82.0                                    toluene   0.0505        66            53.0                                    benzene  0.129          26            66.0                                    ______________________________________                                    

The experimental results reported in Tables 1 to 4, can be discussed inthe following way.

First, as regards the conjugated dienes (see Tables 1 and 2), that is,butadiene and isoprene, it is seen, surprisingly, that themicrostructures obtained are not fundamentally different from thoseobtained in the homogeneous phase by anionic polymerization.

Second, still in the case of conjugated dienes (see Tables 1 and 2), itis found, surprisingly, that the polymers obtained have a majority of1-4 chains, with a relatively low vitreous transition point, which givesthese polymers good elastomeric properties.

Third, still in the case of conjugated dienes (see Tables 1 and 2), itis seen, surprisingly, that, if the results obtained in a saturatedalicyclic solvant (cyclohexane) and in an aromatic solvant (toluene) arecompared for the same monomer, a much larger number of 1-4 chains isobtained and, consequently, the structure tends to be much closer tothat of the natural rubbers, when the solvant is of the aromatic typerather than of the saturated type.

This effect can be attributed a posteriori in the case of an aromaticsolvant to the existence of a ternary graphite/lithium/solvant insertioncompound. The existence such compounds can be checked by allowing thegraphite-lithium binary insertion compound only to act on the solvantand making X-ray diffraction pictures from samples of the insertioncompounds thus modified, for example with a copper or cobaltanti-cathode. If these pictures are compared with the diffractionpictures of the binary compound taken initially, it is seen that newerdiffraction lines have appeared, showing that the structure of theinitial insertion compound has changed. Moreover, by an elementaryanalysis of the modified insertion compound, it can be shown that theempirical formula of the starting insertion compound has, in fact,changed.

But, the insertion of an aromatic solvent into the interior of thecrystal lattice of the graphite leads graphitic an increase in thedistance between the graphite planes. This, a posteriori, can thenexplain the results obtained in the case of the dienes dissolved in anaromatic solvant; for, the monomers can then penetrate the interior ofthe graphitic structure more easily.

Fourth, still in regard to conjugated dienes, by examining thechromatographic patterns with permeable gel of the polymers obtained,that is, by examining the polydispersity curves of these latter, it isseen, surprisingly, that the distribution of the molecular masses of thepolymers prepared according to the invention is relatively narrow, bycomparison with the results obtained generally in heterogeneouscatalysis polymerizations.

Fifth, still in regard to the conjugated dienes, it is seen, byexamining Table 4, that when the quantity of catalyst increases, thereaction time to obtain an equivalent, or even greater yield,diminishes.

Sixth, in the case of the styrene-isoprene copolymerization (see Table3) it is seen, surprisingly, that statistical copolymers are obtained,very near to alternations, in accordance with the followingobservations:

a. the polymerization yields, with respect to styrene and isoprene arecomparable, which indicates an alternation of the polymer obtained,

b. the copolymers obtained have only one vitreous transition point; theyare, therefore, not sequential copolymers,

c. NMR analysis, with 128 spectrum accumulations, shows a single, veryfine, resonance band for the protons (p + m + o) of the aromatic nucleusof styrene; this proves that the mean length of the styrene sequences isvery small and always below 5 (from 5 onward, an o proton peak begins toappear in the NMR spectrum),

d. in addition, the higher percentage of styrene in the macromolecularchain indicates a tendency toward alternation of the isoprene andstyrene patterns; this is confirmed by the abnormally low transitionpoint of the polymer that is obtained, with respect to a same percentageof styrene; for it is known that the existence of an alternatingstructure diminishes the interactions between the styrene units in thesame chain and increases flexibility.

This result is surprising insofar that it is known that in a homogeneousphase and in a non-polar medium, isoprene polymerizes preferentiallywith respect to styrene, so that under these conditions sequentialcopolymers can only be obtained.

What we claim is:
 1. A heterogeneous anionic polymerization process,wherein a reaction medium is formed and comprises firstly at least onemonomer having a conjugated diene structure, secondly at least onecatalyst comprising a carbon with a graphitic structure and at least oneelement inserted in the latter, that is lithium, and thirdly an aromatichydrocarbon.
 2. A process according to claim 2, wherein the catalystused is a ternary insertion compound that comprises the carbon with agraphitic structure, the lithium inserted in the graphitic structure,and the aromatic hydrocarbon also inserted in the latter.
 3. A processaccording to claim 2, wherein the ternary insertion compound is obtainedindependently of the reaction medium, and introduced into the latter. 4.A process according to claim 2, wherein the ternary insertion compoundis obtained "in situ", inside the reaction medium, by bringing withinsaid reaction medium a graphitic carbon/lithium binary insertioncompound into contact with the aromatic hydrocarbon.
 5. A processaccording to claim 1, wherein the catalyst used is a graphiticcarbon/lithium binary insertion compound.
 6. A process according toclaim 4, wherein the catalyst used is a graphitic carbon/lithium binaryinsertion compound.
 7. A process according to claim 5, wherein thebinary insertion compound is a graphite/lithium binary compound, with anempirical formula of LiC_(6n), with 1≦ n≦
 6. 8. A process according toclaim 6, wherein the binary insertion compound is a graphite/lithiumbinary compound, with an empirical formula of LiC_(6n), with 1≦ n≦
 6. 9.A process according to claim 1, wherein the reaction medium comprises asolvent in which the monomer is dissolved and in which the catalyst isin suspension.
 10. A process according to claim 9, wherein the solvantis different from the aromatic hydrocarbon and is chosen from thefollowing hydrocarbon solvants, that is, a saturated aliphatic solvantwith a straight or branched chain, and a saturated alicyclic solvant.11. A process according to claim 9, wherein the aromatic hydrocarbon isalso dissolved in the solvant, the latter being different from saidaromatic hydrocarbon.
 12. A process according to claim 9, wherein thearomatic hydrocarbon serves as a solvant.